Preparation method of high-purity and high-density silicon carbide ceramic

Abstract

The invention belongs to the field of engineering ceramic materials, and particularly relates to a preparation method of high-purity and high-density silicon carbide ceramic. The method comprises the following steps: 1) taking silicon carbide powder, a sintering aid, a silicon source, a carbon source and a binder as raw materials, mixing until the components are uniform, molding and drying to obtain a silicon carbide ceramic green body; and 2) carrying out glue removal, high-temperature sintering and appearance processing on the ceramic green body to obtain the high-purity and high-density silicon carbide ceramic product. By optimizing the formula and the sintering process and combining the advantages of reactive sintering and high-temperature solid-phase sintering, sufficient sintering driving force is obtained while the adding amount of a sintering aid is reduced, sintering densification of the silicon carbide ceramic is realized under the normal-pressure condition, the purity is guaranteed, and the defects that free silicon is easy to remain in reactive sintering and the process is complex are avoided; the invention provides a technical scheme for preparing the high-performance silicon carbide ceramic in a low-cost and large-scale manner. The purity of the prepared silicon carbide ceramic is gt; the density is gt; the bending strength is higher than 410 MPa, and the composite material has a wide commercial application prospect.

Description

Technical Field

[0001] This invention belongs to the field of engineering ceramic materials, specifically relating to a method for preparing high-purity, high-density silicon carbide ceramics. Background Technology

[0002] Silicon carbide ceramics are a type of non-oxide ceramics with excellent high-temperature resistance, corrosion resistance, oxidation resistance, thermal shock resistance, and outstanding mechanical properties. They are also non-toxic and non-polluting. The constituent elements carbon and silicon are abundant in nature, so they have broad application prospects in aerospace, petrochemical, mechanical components, and military armor.

[0003] However, due to the aforementioned characteristics of silicon carbide, it faces numerous challenges in industrial manufacturing: silicon carbide has an extremely low self-diffusion coefficient, making densification difficult to achieve even at temperatures exceeding 2000℃ when sintering under normal pressure. To obtain high-density silicon carbide ceramics, existing technologies typically employ high-pressure molding, high-temperature pressure sintering, or the addition of large amounts of sintering aids. However, these methods have significant drawbacks: high-pressure molding and sintering equipment are expensive, processes are complex, and the resulting product shapes are limited, resulting in low production capacity, making them unsuitable for large-scale, low-cost production of silicon carbide structural ceramics. Furthermore, to ensure that silicon carbide products can withstand the extreme and harsh working environments of high temperature, strong corrosion, and strong mechanical impact, it is crucial to maintain product purity as much as possible while pursuing high density, avoiding performance degradation due to excessive addition of sintering aids.

[0004] In summary, existing technologies for manufacturing high-purity, high-density silicon carbide ceramics are technically challenging, have high mass production costs, and their overall production capacity is far from meeting the demand for their widespread applications. Furthermore, their average lifespan cost is relatively high. To adapt to the upgrading of industrial production technologies, there is an urgent need to develop a method for the low-cost, high-volume production of high-purity, high-density silicon carbide ceramic products. Summary of the Invention

[0005] The purpose of this invention is to overcome the shortcomings of the prior art and provide a method for preparing high-performance silicon carbide ceramics that can be produced on a large scale at low cost.

[0006] To achieve the above objectives, the present invention provides a method for preparing high-purity, high-density silicon carbide ceramics, characterized by comprising the following steps: 1) Silicon carbide powder, sintering aid, silicon source, carbon source and binder are used as raw materials, mixed until the composition is uniform, and then shaped and dried to obtain silicon carbide ceramic green body; 2) The ceramic green body obtained in step 1) is subjected to debinding, high-temperature sintering and shaping to obtain high-purity and high-density silicon carbide ceramic products.

[0007] Furthermore, in step 1), the silicon carbide powder is obtained by using silicon carbide powder with a single particle size or silicon carbide powder with multiple particle sizes in a gradation.

[0008] Furthermore, in step 1), the sintering aid is boron carbide, and the amount of boron carbide added is 0.5-2% of the mass of silicon carbide powder. Silicon carbide is a ceramic material with extremely strong covalent bonds and a very low self-diffusion coefficient, making it difficult to achieve densification through particle diffusion under normal pressure. Boron carbide (B4C), as a classic and efficient aid in silicon carbide sintering, reacts with the silica on the surface of silicon carbide particles during the high-temperature sintering stage to generate a low-melting-point borosilicate oxide liquid phase. This liquid phase can fill the interparticle gaps, accelerating the migration and rearrangement of silicon carbide particles through liquid-phase mass transfer, promoting green body shrinkage; on the other hand, it reduces the interfacial energy between silicon carbide particles, providing impetus for grain growth and porosity elimination, ultimately achieving green body densification. Boron carbide and silicon carbide both belong to carbide ceramics, sharing highly compatible chemical properties, crystal structures (both covalent crystals), and physical properties (such as high-temperature resistance, corrosion resistance, and mechanical strength). Boron carbide's purity is easily controlled, free from volatile and residual harmful impurities, avoiding the potential introduction of impurity phase contamination by other additives (such as metal oxides). Its coefficient of thermal expansion is close to that of silicon carbide, reducing crack defects caused by thermal stress during sintering and ensuring product structural stability. Boron carbide itself possesses extremely high hardness and strength; adding a small amount will not reduce the mechanical properties of silicon carbide ceramics, but rather further enhances strength by refining the grain size. When the amount of boron carbide added is less than 0.5%, the amount of borosilicate liquid phase generated is insufficient to fully fill the gaps between silicon carbide particles and is also difficult to effectively reduce the interparticle interface energy. At this point, the mass transfer bridging effect of the liquid phase is weak, hindering the diffusion and rearrangement of silicon carbide particles. The green body cannot achieve sufficient shrinkage under normal pressure sintering conditions, resulting in reduced density, excessive porosity, and a significant decrease in mechanical properties and corrosion resistance, failing to meet high-performance requirements. Although boron carbide itself is an inert additive, adding more than 2% will lead to an increase in the amount of residual boron carbide dispersed phase. On the one hand, it will reduce the purity of the product, and excessive impurities may become a weak point in performance. On the other hand, too many boron carbide particles may accumulate at the grain boundaries, resulting in a decrease in grain boundary bonding force, which in turn reduces the bending strength and toughness of the ceramic.

[0009] Furthermore, in step 1), the silicon source is an organosilicon source or an inorganic silicon source, and the carbon source is an organic carbon source or an inorganic carbon source; the mass ratio of silicon and carbon provided by the silicon source and the carbon source is 1:1.5-1:3, and the total amount added, calculated in terms of silicon and carbon, is 1-3% of the mass of silicon carbide powder; the binder is an organic binder, and its addition amount is 0.5-2% of the mass of silicon carbide powder.

[0010] Furthermore, in step 1), the mixing method is dry mixing or wet mixing, and the obtained mixed raw material is powder or mud.

[0011] Furthermore, in step 1), the forming method is dry pressing or extrusion molding; the ceramic green body obtained after forming has a certain strength and is free of moisture after drying; the size of the green body after forming is larger than the size of the target finished product, and is adjusted according to the shrinkage rate after sintering.

[0012] Furthermore, in step 2), the glue removal is carried out under a protective atmosphere, the glue removal temperature is 530-680℃, and the heat preservation time is 1-4h.

[0013] Furthermore, in step 2), the high-temperature sintering is carried out under a protective atmosphere, and the sintering method is atmospheric pressure sintering, which includes two holding stages: the first stage temperature is 1320-1480℃, and the holding time is 0.5-2h; the second stage temperature is 2000-2350℃, and the holding time is 1-4h.

[0014] Furthermore, the external shaping process in step 2) is used to trim the shape and adjust the size, and can be performed after glue removal and sintering.

[0015] Furthermore, the organosilicon source is one or more of silane coupling agents and polysiloxanes, the inorganic silicon source is one or more of silicon powder and silicon dioxide; the organic carbon source is one or more of phenolic resin, sucrose, and carbon powder, the inorganic carbon source is graphite powder; and the organic binder is one or more of polyvinylpyrrolidone, cellulose, and oleic acid.

[0016] The beneficial effects of this invention are: The beneficial effects of this invention are as follows: Significant technological advantages: It adopts atmospheric pressure sintering, eliminating the need for expensive high-pressure equipment. The process is simple, mature, efficient, and stable, and is compatible with various molding methods such as dry pressing and extrusion. It can mass-produce silicon carbide products of different shapes, significantly reducing the total production cost.

[0017] Performance Synergistic Optimization: Through precise control of the formulation and sintering regime, an organic combination of reaction sintering and solid-state sintering was achieved in a single sintering process, solving the technical challenge of simultaneously achieving high density and high purity in silicon carbide ceramics during atmospheric pressure sintering. A small amount of silicon source in the formulation generates a liquid phase to promote shrinkage, which is eventually completely converted into the silicon carbide phase. This leverages the shrinkage-promoting advantages of reaction sintering while avoiding the drawbacks of residual free silicon and complex processes inherent in traditional reaction sintering. Simultaneously, it supplements the sintering driving force and reduces the amount of sintering aids required for high-temperature solid-state sintering.

[0018] The product exhibits superior performance: the finished product contains only trace amounts of residual carbon and boron carbide. The residual carbon is low in content and small in size (nanoscale), and is tightly encapsulated by silicon carbide grains. This prevents oxidation and enhances the mechanical properties of the ceramic through a pinning effect. Boron carbide has similar properties to silicon carbide and is added in small quantities, having minimal impact on product performance. The final silicon carbide ceramic has a purity >97%, a density >95%, and a flexural strength higher than 410 MPa, meeting the requirements of extremely harsh working environments. Attached Figure Description

[0019] Figure 1 This is a cross-sectional scanning electron microscope image of Embodiment 1 of the present invention.

[0020] Figure 2 This is a cross-sectional scanning electron microscope image of Embodiment 2 of the present invention. Detailed Implementation

[0021] The present invention will be further described below with reference to specific embodiments. Example 1 The silicon carbide ceramic prepared in this embodiment has a density of 97.7%, a purity of 97.5%, and a flexural strength of 435 MPa. The specific steps are as follows: 1) Silicon carbide powder (100 mesh + 500 mesh gradation), boron carbide, silicon powder, carbon powder, and polyvinylpyrrolidone are placed in a ball mill jar at a mass ratio of 100:1.2:0.4:0.8:1. Silicon carbide balls and anhydrous ethanol are added and ball milled together. The mass ratio of anhydrous ethanol to dry powder is 1:1. The mixture is stirred at a speed of 250 r / min for 2 hours. After drying the obtained slurry, it is ground and sieved. It is then dry-pressed at a pressure of 60 MPa to obtain a green body (the green body size is 8% larger than the target finished product). The green body is then dried in an oven at 100℃ for 4 hours to ensure that there is no moisture. 2) Place the green blank in a debinding furnace and debind it at 620℃ under a nitrogen atmosphere for 1 hour. Then place the green blank in a sintering furnace and hold it at 1420℃ for 0.5 hours under an argon atmosphere. Then raise the temperature to 2100℃ and hold it for 1 hour. After the holding is completed and the blank is cooled, it is taken out, sintered, ground and trimmed to obtain high-purity and dense silicon carbide ceramic.

[0022] Example 2 The silicon carbide ceramic prepared in this embodiment has a density of 95.6%, a purity of 97.1%, and a flexural strength of 412 MPa. The specific steps are as follows: 1) Silicon carbide powder (single 200 mesh), boron carbide, polysiloxane, phenolic resin, and cellulose are placed into a drum mixer in a mass ratio of 100:1.8:0.6:1.2:1.5. The mixture is first dry-mixed for 40 minutes, then deionized water (mass ratio to dry powder is 0.3:1) is added and mixing continues for 20 minutes. The resulting wet powder is placed in a ply mill for plying. After plying, the mixture is aged at room temperature for 48 hours to obtain a uniformly mixed material. The material is extruded under an extrusion pressure of 5 kg, then placed at room temperature for 12 hours, and subsequently dried in a 90℃ drying oven for 12 hours until the moisture is completely removed, obtaining a green body (the green body size is 10% larger than the target finished product). 2) Place the green blank in a debinding furnace and debind it at 670℃ under a nitrogen atmosphere for 3 hours. After debinding, perform rough processing. Then, place the green blank in a sintering furnace and hold it at 1380℃ for 1 hour under an argon atmosphere. Then, raise the temperature to 2110℃ and hold it for 1.5 hours. After the holding period ends and the blank is cooled, remove it and perform fine processing to obtain high-purity and dense silicon carbide ceramic.

[0023] Example 3 The silicon carbide ceramic prepared in this embodiment has a density of 95.1%, a purity of 98.2%, and a flexural strength of 415 MPa. The specific steps are as follows: 1) Silicon carbide powder (single 100 mesh), boron carbide, silane coupling agent, sucrose, and oleic acid are placed in a mixer at a mass ratio of 100:0.5:0.3:0.9:0.5 and dry-mixed for 60 minutes until the composition is uniform to obtain powder. The powder is dry-pressed at a pressure of 80 MPa to obtain a green body (the green body size is 12% larger than the target finished product), and dried in an oven at 110℃ for 3 hours to ensure no moisture. 2) The green body is placed in a debinding furnace and debinded at 580℃ under an argon atmosphere for 2 hours. Then, the green body is placed in a sintering furnace and held at 1480℃ for 0.5 hours under an argon atmosphere. Subsequently, the temperature is raised to 2350℃ and held for 1 hour. After cooling, it is taken out and its shape is trimmed to obtain high-purity and dense silicon carbide ceramic.

[0024] Comparative Example 1 The silicon carbide ceramic prepared in this comparative example has a density of 88.3%, a purity of 98.0%, and a flexural strength of 320 MPa. The specific steps are as follows: 1) Silicon carbide powder (100 mesh + 500 mesh gradation), boron carbide, and polyvinylpyrrolidone were placed in a ball mill jar at a mass ratio of 100:1.2:1. Silicon carbide balls and anhydrous ethanol were added and ball milled to mix. The mass ratio of anhydrous ethanol to dry powder was 1:1. The mixture was stirred at a speed of 250 r / min for 2 h. After drying the obtained slurry, it was ground and sieved. It was then dry-pressed at a pressure of 60 MPa to obtain a green body (the green body size was 8% larger than the target finished product). The green body was then dried in an oven at 100℃ for 4 h to ensure that there was no moisture. 2) Place the green blank in a debinding furnace and debind it at 620℃ under a nitrogen atmosphere for 1 hour. Then place the green blank in a sintering furnace and hold it at 1420℃ for 0.5 hours under an argon atmosphere. Then raise the temperature to 2100℃ and hold it for 1 hour. After the holding is completed and the blank is cooled, it is taken out, sintered, ground and trimmed to obtain silicon carbide ceramic products.

[0025] Comparative Example 2 The silicon carbide ceramic prepared in this comparative example has a density of 96.0%, a purity of 95.2%, and a flexural strength of 380 MPa. The specific steps are as follows: 1) Silicon carbide powder (100 mesh + 500 mesh gradation), boron carbide, silicon powder, carbon powder, and polyvinylpyrrolidone were placed in a ball mill jar at a mass ratio of 100:3.0:0.4:0.8:1. Silicon carbide balls and anhydrous ethanol were added and ball milled together. The mass ratio of anhydrous ethanol to dry powder was 1:1. The mixture was stirred at a speed of 250 r / min for 2 h. After drying the obtained slurry, it was ground and sieved. It was then dry-pressed at a pressure of 60 MPa to obtain a green body (the green body size was 8% larger than the target finished product). The green body was then dried in an oven at 100℃ for 4 h to ensure that there was no moisture. 2) Place the green blank in a debinding furnace and debind it at 620℃ under a nitrogen atmosphere for 1 hour. Then place the green blank in a sintering furnace and hold it at 1420℃ for 0.5 hours under an argon atmosphere. Then raise the temperature to 2100℃ and hold it for 1 hour. After the holding is completed and the blank is cooled, it is taken out, sintered, ground and trimmed to obtain silicon carbide ceramic products.

[0026] Comparative Example 3 The silicon carbide ceramic prepared in this comparative example has a density of 90.1%, a purity of 97.5%, and a flexural strength of 350 MPa. The specific steps are as follows: 1) Silicon carbide powder (100 mesh + 500 mesh gradation), boron carbide, silicon powder, carbon powder, and polyvinylpyrrolidone were placed in a ball mill jar at a mass ratio of 100:1.2:0.4:0.8:1. Silicon carbide balls and anhydrous ethanol were added and ball milled together. The mass ratio of anhydrous ethanol to dry powder was 1:1. The mixture was stirred at a speed of 250 r / min for 2 h. After drying the obtained slurry, it was ground and sieved. It was then dry-pressed at a pressure of 60 MPa to obtain a green body (the green body size was 8% larger than the target finished product). The green body was then dried in an oven at 100℃ for 4 h to ensure that there was no moisture. 2) Place the green blank in a debinding furnace and debind it at 620℃ under a nitrogen atmosphere for 1 hour. Then place the green blank in a sintering furnace and heat it directly to 2100℃ under an argon atmosphere for 1.5 hours. After the holding period ends and it is cooled, it is taken out, sintered, ground and trimmed to obtain silicon carbide ceramic products.

[0027] Comparative Example 4 The silicon carbide ceramic prepared in this comparative example has a density of 96.2%, a purity of 94.3%, and a flexural strength of 400 MPa. The specific steps are as follows: 1) Silicon carbide powder (single 200 mesh), alumina-yttrium oxide (traditional additive), and polyvinylpyrrolidone are placed in a ball mill jar at a mass ratio of 100:5.0:1. Silicon carbide balls and anhydrous ethanol are added and ball milled to mix. The mass ratio of anhydrous ethanol to dry powder is 1:1. The mixture is stirred at a speed of 250 r / min for 2 h. After drying the obtained slurry, it is ground and sieved. It is then dry-pressed at a pressure of 60 MPa to obtain a green body (the green body size is 8% larger than the target finished product). The green body is then dried in an oven at 100℃ for 4 h to ensure that there is no moisture. 2) Place the green blank in a debinding furnace and debind it at 620℃ under a nitrogen atmosphere for 1 hour. Then place the green blank in a hot pressing sintering furnace and hold it at 2100℃ for 1.5 hours under an argon atmosphere and a pressure of 20MPa. After the holding period ends and the blank is cooled, it is taken out, sintered, ground and trimmed to obtain silicon carbide ceramic products.

[0028] Comparative Example 5 The silicon carbide ceramic prepared in this comparative example has a density of 92.5%, a purity of 96.8%, and a flexural strength of 370 MPa. The specific steps are as follows: 1) Silicon carbide powder (100 mesh + 500 mesh gradation), boron carbide, silicon powder, carbon powder, and polyvinylpyrrolidone were placed in a ball mill jar at a mass ratio of 100:1.2:1.0:0.5:1. Silicon carbide balls and anhydrous ethanol were added and ball milled together. The mass ratio of anhydrous ethanol to dry powder was 1:1. The mixture was stirred at a speed of 250 r / min for 2 h. After drying the obtained slurry, it was ground and sieved. It was then dry-pressed at a pressure of 60 MPa to obtain a green body (the green body size was 8% larger than the target finished product). The green body was then dried in an oven at 100℃ for 4 h to ensure that there was no moisture. 2) Place the green blank in a debinding furnace and debind it at 620℃ under a nitrogen atmosphere for 1 hour. Then place the green blank in a sintering furnace and hold it at 1420℃ for 0.5 hours under an argon atmosphere. Then raise the temperature to 2100℃ and hold it for 1 hour. After the holding is completed and the blank is cooled, it is taken out, sintered, ground and trimmed to obtain silicon carbide ceramic products.

[0029] Through the above examples and comparative examples, it can be seen that by using a boron carbide (0.5-2%) combined with a silicon carbide source (1-3%, mass ratio 1:1.5-3) formulation and a two-stage atmospheric pressure sintering process, the silicon carbide ceramics prepared all achieve high purity, high density, and high mechanical properties synergistically, with purity >97%, density >95%, and flexural strength >410MPa. The highest density reaches 97.7% and flexural strength 435MPa, which confirms the compatibility between the formulation and the process. In the comparative examples, abnormal core conditions all led to significant performance degradation: No silicon-carbon source: density only 88.3%, flexural strength 320 MPa, insufficient densification and sintering driving force; Excess boron carbide (3%): purity reduced to 95.2%, flexural strength 380 MPa, excessive additives introduced impurities and weakened grain boundary bonding; Omitting the first stage of sintering and holding: density 90.1%, flexural strength 350 MPa, insufficient liquid phase formation and reaction; Traditional additives (alumina-yttrium oxide) combined with hot pressing sintering: purity 94.3%, flexural strength 400 MPa, decreased purity and dependence on high-pressure equipment; Imbalanced silicon-carbon mass ratio (1:0.5): density 92.5%, flexural strength 370 MPa, weakened reaction sintering effect, insufficient densification.

[0030] The embodiments described above are some, but not all, embodiments of the present invention. The detailed description of the embodiments of the present invention is not intended to limit the scope of the claimed invention, but merely to illustrate selected embodiments. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort are within the scope of protection of the present invention.

Claims

1. A method for preparing high-purity, high-density silicon carbide ceramics, characterized in that, Includes the following steps: 1) Silicon carbide powder, sintering aid, silicon source, carbon source and binder are used as raw materials, mixed until the composition is uniform, and then shaped and dried to obtain silicon carbide ceramic green body; 2) The ceramic green body obtained in step 1) is subjected to debinding, high-temperature sintering and shaping to obtain high-purity and high-density silicon carbide ceramic products.

2. The preparation method according to claim 1, characterized in that, In step 1), the silicon carbide powder is obtained by using silicon carbide powder with a single particle size or silicon carbide powder with multiple particle sizes in a gradation.

3. The preparation method according to claim 1, characterized in that, In step 1), the sintering aid is boron carbide, and the amount of boron carbide added is 0.5-2% of the mass of silicon carbide powder.

4. The preparation method according to claim 1, characterized in that, In step 1), the silicon source is an organosilicon source or an inorganic silicon source, and the carbon source is an organic carbon source or an inorganic carbon source; the mass ratio of silicon and carbon provided by the silicon source and carbon source is 1:1.5-1:3, and the total amount added based on silicon and carbon is 1-3% of the mass of silicon carbide powder; the binder is an organic binder, and its addition amount is 0.5-2% of the mass of silicon carbide powder.

5. The preparation method according to claim 1, characterized in that, In step 1), the mixing method is either dry mixing or wet mixing, and the resulting mixed raw material is powder or mud.

6. The preparation method according to claim 1, characterized in that, In step 1), the forming method is dry pressing or extrusion molding; the ceramic green body obtained after forming has a certain strength and is free of moisture after drying; the size of the green body after forming is larger than the size of the target finished product, and is adjusted according to the shrinkage rate after sintering.

7. The preparation method according to claim 1, characterized in that, In step 2), the glue removal is carried out under a protective atmosphere, with a removal temperature of 530-680℃ and a holding time of 1-4 hours.

8. The preparation method according to claim 1, characterized in that, In step 2), the high-temperature sintering is carried out under a protective atmosphere and the sintering method is atmospheric pressure sintering, which includes two holding stages: the first stage temperature is 1320-1480℃ and the holding time is 0.5-2h; the second stage temperature is 2000-2350℃ and the holding time is 1-4h.

9. The preparation method according to claim 1, characterized in that, In step 2), the external shaping is used to trim the shape and adjust the size. This process can be performed after glue removal and after sintering.

10. The preparation method according to claim 4, characterized in that, The organosilicon source is one or more of silane coupling agents and polysiloxanes; the inorganic silicon source is one or more of silicon powder and silicon dioxide; the organic carbon source is one or more of phenolic resin, sucrose, and carbon powder; the inorganic carbon source is graphite powder; and the organic binder is one or more of polyvinylpyrrolidone, cellulose, and oleic acid.

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